I read about the preference of a large turbo housing with a small turbine impellor for HP and cooler exhaust but I just don't get the concept !? How about some turbo talk to straighten me out! STY

 

With the larger housing, you can push MORE exhaust through easier so it spools faster and keeps it cooler, and the smaller impeller shaft is lighter so it spools faster. At least that's my understanding on it.

 

For more boost pressure you want a larger INTAKE housing and/or a smaller EXHAUST housing depending on your situation. The theory is kinda like a garden hose...turn the water on and it flows at a certain speed w/ a certain gal/min value. Put your thumb over the end of it and you've made the opening smaller, but the water still wants to flow at its predetermined pressure (via the shutoff valve) so it flows FASTER.

Now, imagine that the exhaust housing is your garden hose. If you make the housing smaller the exhaust is gonna run through it faster than if it's bigger, which will turn the impeller FASTER. Let's say you can get 30 psi of boost pressure from your stock intake housing. If the intake housing is bigger w/ an impeller that's turning faster, you can get more volume of air at that same pressure into your cylinders. But you see, THAT'S the key...you need to be able to turn the impeller faster than your present turbo set-up to make a difference by getting a bigger intake housing. If you get a smaller exhaust housing your impeller will turn faster and you might be able to stick w/ the stock intake housing. Either way, it's quite the complicated ordeal and is all related to how fast can you move a certain amount of air using your exhaust to power the fan.

 

OK, so the bigger housing allows more exhaust to flow easyier which in turn removes heat quicker with less resistance, right?
Same idea with a smaller impellor, less resistance so it spins easier and comes up to rpm quicker, is this right so far?
Now, how does that relate to increased HP or Performance when the compressed intake air is apparantly reduced by using a smaller impellor? Isn't more air needed for more fuel to make more power, hence twin turbos?

STY

 

Welder 27, I replied before reading your post. It sounds like that makes more sence for building performance by boosting more combustion air and restricting the "garden hose" to apply more pressure to the impellor . Now how does that relate to decreased exhaust temps. when the restriction at the exhaust [smaller] is retarding the escape of the eg.?


Over 50 and still not to good at relationships I guess! STY

 

Well, that's where a 4" or 5" straight pipe from the turbo all the way out comes into play. Essentially what you create is the Venturi effect...if you connect a garden hose to a smaller hose and then back to a garden hose you should (in theory) have the same amount of water flow at the end, but the water will flow a lot faster in the smaller part of the hose. Actually, I don't think that's a good analogy, because water isn't compressible. Picture an air hose w/ that same set-up. The air in the smaller section of pipe will be compressed to a higher pressure AND it will be flowing faster. Then when that air gets through the restricted section it will return to its original pressure and speed.

That's why it's so important to get a bigger exhaust pipe whenever you do any BOMBing. If you don't you risk destroying your engine w/ excessive EGT's and backpressure. Don't get me wrong, you'll always have some amount of backpressure...but you just have to figure out how much is too much!

 

OK so with the venturi principle , the pressure drop on the outlet side of the turbo to 4 inch connection effectivly causes a temperature drop as well, yes? But that temp. drop is post turbo, allowing that the restricted turbo outlet increases pressure, and therefore temp. then it reasons that pre turbo temp would be increaswed in this application of bigger air side/smaller exhaust side , smaller impellor turbo set up? sty

 

I'm pretty sure the Venturi effect is like this...say 50 psi @ 100 mph @ 1000*F pre turbo, 60 psi @ 150 mph @ 1200*F inside turbo, 50 psi @ 100 mph @ 1000*F post turbo. That's what I'm thinking. I just pulled those numbers out of the air to show what I mean...they aren't actual.

Anytime air is compressed at a fast rate its temperature goes up. They say the air on the intake side of the turbo can reach 30 psi and 300*F...that's why most turbo diesel trucks have an intercooler...cold air is more dense thus has more oxygen.

 

Thanks for the input, welder. I''ll chew on it a while more and see what I can swallow.......sty

 

Here is a good place to get some info on the turbos on our trucks.
http://www.holset.co.uk/flash/index.html

 

Welder27, You make some good points but I don't think your Garden hose analogy is accurate (If I understand what you are saying).
IMO, you never want to restrict flow on the turbine (exhaust) side. Look at twins...Exhaust goes from engine to small turbo...to large turbo...to 5" exhaust pipe. Always expanding. Air side is the opposite..from big turbo...to small turbo.. to air intake.
I think the answer to smaller vs larger turbine wheel is simply speed of spool up. Smaller spools sooner for any given amount of exhaust gas.
All turbos are a trade off - faster spool up hurt you on the top (high speed) end. Bigger turbine wheel takes longer to spoolup, giving more smoke and delayed response but is more efficient at high speed.
Rangerst - Larger compressor housing (air side) moves more air and cooler air (denser) thus more hp and lower EGT's.
Balancing act is between the size of both wheels and the end result desired.
Hope my butting in this late in the conversation helps, or at least does not add to the confusion.
Rowland

 

Thanks RJ, I'll keep chewin'!...sty

 

rjohnson, I see what you're sayin...water isn't compressible, so that's not an accurate analogy. And it is a balancing act...it all really depends on what your engine will put out and how your fuel pump is set up. A drag racing turbo definitely wouldn't be the choice for heavy hauling and vice-versa!

 

"OK, so the bigger housing allows more exhaust to flow easyier which in turn removes heat quicker with less resistance, right?
Same idea with a smaller impellor, less resistance so it spins easier and comes up to rpm quicker, is this right so far?
Now, how does that relate to increased HP or Performance when the compressed intake air is apparantly reduced by using a smaller impellor? Isn't more air needed for more fuel to make more power, hence twin turbos?"
_________________________________________
This is great discussion. I, like you, have "chewed" on this for a long time. Read 2 books on turbo and still have trouble being clear enough about it to put understanding into words.

All you say above is true, as I understand it. What might help is this.
With Diesels - more fuel = more power, period. The key is to burn the additional fuel. Hence, need for more air. With any given combination of injectors and turbo there is only one throttle position (hp level) where the entire system is balanced (all air used and all fuel burned). When system is "balanced" EGT's will be at lowest possible for that hp level, on that truck.

Problems arise due to driving at many different throttle positions. Hence the balancing act and the search for best balance of fuel/air for each type of driving.
Finding balance gets more complicated by fairly narrow band of efficiency for any turbos air output. To burn fuel need not just more volume of air, but more cool (dense) air. Spin a turbo too fast and you get more volume of air, but it's hot (thin) air- and less fuel actually gets burned. "Hence twin turbos". Twins give more volume of air through compounding effect, AND it's all cool, dense air, due to lower RPM of each turbo.
Hope this info helps you figure it out!
Rowland

 

Exhaust:
smaller housing = quicker spool/higher EGT ( you can only push so much exhaust through it before it exceeds efficiency map)
larger housing = slower spool/lower EGT (takes longer to spool unless heavily fueled, can smoke)

Intake:
smaller housing = lower pressure/higher EGT (due to less volume/efficiency)
larger housing = higher pressure/lower EGT (due to greater volume/effeciency)

The above of course is from a stock starting point.

Each turbo can only compress so much air before it exceeds is volumetric efficiency and starts to pump hot air into the engine thus causing EGT's to rise. So choosing a turbo is a balancing act between quick spooling and top end pressure/volume. This is where the hybrid turbos really shine.